Compositions and methods for determining directionality of radiation

ABSTRACT

A method of determining directionality of radiation is disclosed which comprises dividing the tensioned metastable fluid liquid volume adjacent to a radioactive source into a plurality of sectors, determining the opposing sector ratio of the respective sector and determining the direction of the radiation based on the opposing sector ratios of the plurality of sectors. The method further comprising determining directionality of incoming radiation from the tension pressure assisted elongation of bubble shapes pointing towards direction of radiation particles that interacted with nuclei of tensioned metastable fluid detector system. A device capable of carrying out these methods is also disclosed.

PRIORITY

This application claims the benefit of U.S. Provisional Application No.61/174,159 filed Apr. 30, 2009, the entire contents of which areincorporated herein by reference.

INVENTION MADE WITH U.S. GOVERNMENT SUPPORT

This invention was made with Government Support under Contract No.HR0011-05-C-0141 awarded by the Defense Advanced Projects ResearchAgency. The Government has certain rights in this invention.

TECHNICAL FIELD

The present disclosure relates to a method of determining directionalityof radiation. More specifically, the present disclosure relates to amethod of determining directionality of radiation using a tensionedmetastable fluid detection system.

BACKGROUND

Radiation cannot be detected by human senses. A variety of handheld andlaboratory instruments is available for detecting and measuringradiation, such as Geiger counters. However, these devices do notprovide information about the direction from which the radiationemanates.

BRIEF SUMMARY

This disclosure provides compositions and methods for determining thedirection of incoming radiation.

One method of determining directionality of radiation involves dividinga tensioned metastable volume of fluid in a chamber into a plurality ofsectors, placing the fluid in the proximity of a radiation source anddetecting radiation induced cavitation nucleation events in variousregions or sectors within the chamber, determining the opposing sectorratio of the number of cavitations in the respective sectors anddetermining the direction of the radioactive source based on theopposing sector cavitation ratios in the plurality of sectors.

A neutron detection system is disclosed that has the ability to givedirectional information about the source. Rather than relying on neutronor other radiation (e.g., photon) interactions that give no directionalinformation about the origin of the radiation being detected, anacoustic tensioned metastable fluid detection (ATMFD) system can be usedto show which direction the radiation is coming from.

While the ATMFD system is operating, the probability that aneutron/radiation induced cavitation event will occur is a function ofthe tensioned or negative pressure in the fluid and theneutron/radiation flux. One embodiment relies upon an acoustic tensionedmetastable fluid in which the pressure profile is nearly axiallysymmetric on a horizontal plane such that all points that are equallydistant from the center will have substantially the same negativepressure. In such a system the cavitation probability is a function ofneutron/radiation flux. Since neutron/radiation flux from a sourcedecreases with distance and with the degree of down scattering andabsorption, the side sector of the detector closest to the source has ahigher probability of detection. By detecting the location of a sampleset of cavitation events, directional information can be determined byobserving an imbalance in the locations of cavitation events.

The location of a neutron/radiation detection nucleation site can bedetermined by recording the time at which the resulting cavitationinduced shockwave reaches various locations on the detector wall. Anynumber of transducers can be incorporated into the detector in order todetermine the location of the source of cavitation induced shock waves.Any suitable number of transducers can be used so long as directionalinformation can be obtained. For example, four (approximately 7 mm OD)piezoelectric transducers can be used to detect the arrival of theshockwave from the cavitation event. At least two, preferably three ormore of the signal detection transducers can be in a plane and one ormore signal detection transducers can be outside that plane. The signalsfrom the four transducers can then be processed to derive desiredinformation on directionality. Additionally, directional information mayalso be obtained from monitoring of the bubble shapes at the time, andafter cavitation events occur. Such cavitation bubbles generated fromneutron/radiation strike on to nuclei of atoms in the acousticallytensioned pressure field of the ATMFD liquid preferentially extendthemselves in elliptical like shapes pointing in the direction ofincoming radiation.

The preferred ATMFD systems described herein have the ability to:

-   -   Detect SNM neutrons over eight orders of magnitude,    -   Detect alpha particles,    -   Maintain virtually complete insensitivity to gamma photons,    -   Operate with intrinsic efficiency of about 90%,    -   Provide real-time directional information of incoming radiation.

Benchmarking and qualification studies have been conducted with Pu-basedneutron-gamma and photon light sources. This disclosure provides themodeling cum-experimental framework along with a demonstration of theoperation of the ATMFD system.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides schematic views of ATMFD resonant chambers (a) havingmultiple disk transducers positioned with the hollow cylinder transduceror (b) separately with multiple transducers positioned spaced apart.

FIG. 2 is a schematic view of pressure distributions in a chamber atabout 4.5 W and about 10 W.

FIG. 3 is a not-to-scale view of the geometry of an MCNP input deck.

FIG. 4 is a schematic view of neutron flux ratio at large standoff foracetone and Freon-113.

FIG. 5 is a schematic view of the screenshot of shock pulses beforesignal processing. The vertical scale is about 500 mV/div. Thehorizontal scale is about 100 ms/div.

FIG. 6 is a schematic view of a screenshot of shock pulses analyzed fordirectionality information. The vertical scale is about 500 mV/div. Thehorizontal scale is about 5 μs/div.

FIG. 7 is a schematic view of the experimental setup with directionalityautomation.

FIG. 8 is an axial cross-section view of positions of detection eventsusing a PuBe source about −20.3 cm away from the center axis of chamberin line with Mic 1 and Mic 3.

FIG. 9 is a schematic view of the determination of source directionwithin ±30°. Comparison of experimental results with MCNP simulation.

FIG. 10 is a schematic view of all neutron detection events as recorded,seen in rz plane, overlaid with computer software (COMSOL®) simulationsat about 4.5 W.

FIG. 11 is a schematic view of pressure distributions in chamber about4.5 W (a) and about 10 W (b).

FIG. 12 is a not-to-scale view of the geometry of MCNP input.

FIG. 13 is a schematic view of the oscilloscope trace of cavitationshock waves. The vertical scale of first signal is about 20 mV and thesecond signal is about 200 mV. The horizontal scale is about 50 ms/div.

FIG. 14 is a schematic view of the screenshot of shock pulses analyzedfor directionality information.

FIG. 15 is a schematic view of the trigger rate recorded by eachtransducer for various triggering levels.

FIG. 16 is a schematic view of the experimental setup withdirectionality automation.

FIG. 17 is a schematic view of data taken manually with 4 channel 100MHz 100 MSa/s oscilloscope.

FIG. 18 is a schematic view of positions of cavitation events using PuBesource about −35.5 cm away from center axis of chamber in line with Mic1 and Mic 3.

FIG. 19 is a schematic view of positions of cavitation events using PuBesource about +35.5 cm away from the center axis of the chamber in linewith Mic 1 and Mic 3.

FIG. 20 is a schematic view of all cavitation events as recorded, seenin the xz plane. Data taken with source about −35.5 cm and 35.5 cm awayfrom the center of chamber on x-axis with Mics 1 and 3.

FIG. 21 is a schematic view of radial distribution of cavitation events,separated into two sections (closest to source and furthest fromsource); (a) data taken with source about −35.5 cm away from the centerof the chamber on the x-axis; (b) data taken with source about 35.5 cmaway from the center of the chamber on the x-axis.

FIG. 22 is a picture of an elongated bubble in an ATMFD pointing to thesource of incoming (in this case neutrons from a Pu—Be isotopic source)radiation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one method the capacity to detect nuclear particles can be providedby tensioned metastable fluid states which can be attained via tailoredresonant acoustic systems such as acoustic tensioned metastable fluiddetector (ATMFD). Radiation detection in tensioned metastable fluids canbe accomplished via macro-mechanical manifestations derived from thefemtoscale nuclear interactions. Incident nuclear particles can interactwith the dynamically tensioned metastable fluid wherein theintermolecular bonds are sufficiently weakened such that the recoil ofionized nuclei generates nano-scale vapor cavities which grow to visiblescales. Ionized nuclei form preferentially closest to the incomingradiation, thereby providing for the first time the capability toascertain information on directionality of incoming radiation.

The present disclosure provides advancements in the detection of abroader range of nuclear particles, the detection of neutrons over anenergy range of eight orders of magnitude, improved intrinsic detectionefficiencies beyond 90%, and in ascertaining directionality informationof incoming radiation than has previously been possible. In one example,the present disclosure provides a composition and method which increasesthe accuracy and precision of ascertaining directionality informationutilizing enhanced signal processing cum-signal analysis, refinedcomputational algorithms, and on demand enlargement of the detectorsensitive volume.

Advances in the development of ATMFD systems can be accomplished throughthe use of a combination of experimental and theoretical modeling.Modeling methodologies include Monte-Carlo based nuclear particletransport using MCNP5 and complex multi-physics based assessmentsaccounting for acoustic, structural, and electromagnetic coupling of theATMFD system via a computer software (COMSOL®) multiphysics simulationplatform. Benchmarking and qualification studies have been conductedwith special nuclear materials (SNMs), including Pu-based neutron-gammasources. The results show that the ATMFD system, in its currentconfiguration, is capable of locating the direction of a radioactivesource at least to within about 30° with about 80% confidence or more.

A First Embodiment

Radiation detection in tensioned metastable fluids is based, in part, onthe principle that incident nuclear particles interact with a tensionedfluid wherein the intermolecular bonds are sufficiently weakened suchthat nuclear particles are capable of triggering a localized explosivephase change in the fluid. A liquid in a tensile state is metastablebelow its thermal equilibrium state, unlike a superheated liquid whichis in a state of thermal superheat which is above its normal boilingpoint. Tension in fluids is analogous to the stretching of solidstructures. The energy required to tear apart the intermolecular bondsof a solid decreases as the tension in the structure increases. In ananalogous manner, the excess trigger energy required to break theintermolecular bonds between liquid molecules decreases with increasingtension metastability; eventually resulting in spontaneous triggering ofexplosive phase change at the spinodal limit of tension. Below thisstability limit, excess energy is required to trigger phase change ofthe tensioned metastable fluid. This excess energy can be provided viainteraction with nuclear particles (e.g., neutrons, alphas, photons,betas, fission products, etc.) or even with visible light photons. Thisproperty enables the amplification of femto-scale nuclear scaleparticles to relatively large (×10¹³) macroscopic scales thereforeallowing for new low-cost, ultra-sensitive detectors for nuclearengineering and scientific applications such as the acousticallytensioned metastable fluid based detection system (ATMFD) describedherein.

This disclosure is directed to the directional detection of neutrons inthe MeV range (e.g., those neutrons emitted by special nuclear materialssuch as U, Pu, Cf, Am, Cm, etc.) via direct knock-on collisions with thedetector fluid.

The ATMFD approach appears capable of not only detecting the energy andintensity of incident radiation, but also ascertaining information onthe location of the radioactive source, a feature of significantpotential use in widespread fields, including identifying the tell-taleneutron emission signatures from SNMs for homeland security. Directionalinformation is ascertainable in the ATMFD system due to the increasedprobability that a neutron induced detection event will occur in theregional sector of the tensioned fluid volume nearest the source. Theprobability that a neutron induced detection event will occur is afunction of the negative pressure in the detector fluid and the neutronflux and energy. Since the pressure profile is nearly axi-symmetric, theprobability of detection events is a function of the neutron flux andenergy. Since neutron flux of a given energy from a source decreaseswith distance and with the degree of down scattering and absorption, theside of the sensitive, volume nearest the source has the highestprobability of detection. Detecting the location of these detectionevents inside the detector allows the user to ascertain information onthe direction of the radioactive source. The present disclosure providesan improved mechanistic treatment of directionality determination.

Nomenclature

-   -   ATMFD—Acoustic tension metastable fluid detector

COMSOL®—COMSOL® multiphysics computer simulation platform

-   -   GPIB—General purpose interface bus (IEEE 488)    -   GPS—Global positioning system

LABVIEW®—computer software for industrial automation and scientific andengineering data acquisition, control and analysis

-   -   LET—Linear energy transfer (also dE/dx)    -   PuBe—Plutonium beryllium neutron source    -   PZT—Lead zirconate titanate    -   Mic—Microphone    -   MCNP5—Monte Carlo n-Particle Version 5    -   OD—Outer diameter    -   SDD—Superheated droplet detector    -   SNM—Special nuclear material    -   TDOA—Time difference of arrival (also 't)    -   TMFD—Tension metastable fluid detector    -   V1—Sensitive volume (or sector) nearest the source    -   V2—Sensitive volume (or sector) farthest from source    -   XatMaxY—Time in of the highest peak relative to the trigger        point.    -   XatMinY—Time in of the lowest peak relative to the trigger point

ATMFD Design

Any suitable fluid chamber can be used so long as the chamber can beused to create a tensioned metastable fluid, preferably an acoustictensioned metastable fluid, in which a fluid pressure profile can becreated that is nearly axially symmetric such that all points that areequally distant from the central axis will have substantially the samenegative pressure, a cylinder for example. Due to manufacturing issues,cylinders of glass can involve slight (˜40-100 micron type) deviationsin thickness and diameter along the circumference and length. As aconsequence, the true central axis of a resonance chamber can shift fromthe centerline. Such a shift can create asymmetry in the oscillatingpressure profiles in the radial and axial directions. Such variationscan be accounted for up-front by system system characterization. Forexample by transient oscillating pressure mapping over a range offrequencies of interest to find the true central axis. For practicalsystems, the fluctuating pressures from the geometrical central axiswill generally be substantially the same, albeit, somewhat skewed but toa known level such that adjustments can be made when derivingdirectionality related information. In certain embodiments suitablechambers will have the characteristic that they can be mechanicallydeformed in transient fashion from pulses from an external transducer,in a manner that generates a standing acoustic wave in the fluid housedby the chamber. The chamber in certain embodiments has a size and shapethat allows for the directional detection of radiation that permits downscattering assisted collection of bubble cavitation events in variousregions of the chamber. The pressure wave can consist of oscillatingpositive and negative pressures, such that the negative pressures are ina range above the spinodal limit of tension but which allows the energyreleased by interaction of nuclear particles with fluid molecules totrigger a phase change also known as a bubble nucleation or cavitationevent. Generally, when freon-113 is the fluid, the negative pressure isthought to be about −2.5 bar or lower in the presence of ^(˜)4 MeVneutrons from an SNM such as from a Pu—Be source. Where acetone is thetest liquid the negative pressure is about −3.5 bar or lower. Therequired negative pressure is variable with the external neutron energyand can be calibrated a priori by comparing against neutron sources ofknown energy (e.g., from accelerator systems or mixtures of alphaemitting isotopic sources such as Am—Be, Am—Li, Am—B, Am—C, Am—Fl andthe like). Suitable chambers can be manufactured from quartz, glass(preferably Pyrex glass), ceramics, polycarbonates, and a number ofmetals, as is known in the art. In one embodiment, a resonant acousticchamber can have an outside diameter of approximately 70 mm and 150 mmlong cylindrical quartz tube having a hemispherical top and bottom. Aschematic of this ATMFD is shown in FIG. 1 a. Other dimensions can bechosen as well to fit needs related to frequency of operation. Thechamber can be filled with a fluid and is generally sealed. The chambercan be adapted with a mechanism for focusing acoustic energy within thefluid in the chamber. Acoustic energy can be focused within the fluidinside the chamber by any suitable means, for example a hollow glass orquartz reflector placed at the top of the test fluid and a similarhollow glass or quartz reflector placed at the bottom of the chamber canbe used. Plastics, TEFLON® polytetrafluoroethylenes (PTFE) orpolycarbonates may be used if they are not attacked chemically by theworking fluid. A concentrically ring shaped piezoelectric transducermade of lead-zirconate titanate (PZT) can be affixed by standard methods(mechanical or epoxy glue-based) to the outside of the chamber and usedto power the acoustic resonance chamber. Suitable transducers can bemade of any material that can induce acoustic resonance within thefluid, suitable materials include ceramic materials such as bariumtitanate, lead zirconate titanate (PZT), among other materials, as areknown. It is not necessary to use a concentric ring shaped hollowcylinder. This is especially true for large diameter ATMFDs where largecircular concentric ring transducers become increasingly more difficultto procure. As an alternate, multiple disks such as circular,rectangular or other shaped transducers may be positioned as shownschematically in FIG. 1 b either together with the hollow cylinder as inFIG. 1 a, or by themselves as shown in FIG. 1 b. In such a situation,about 4 such disks are positioned in a given plane and act as drivetransducers. The fifth is mounted at a higher elevation and may be ofsignificant smaller size—the purpose being to receive shock signals. Thefour in the same plane act to not only provide drive power but also toreceive shock signals from imploding bubbles. In both cases, thethickness and size for a given material control the capacitance andresonance frequency of the transducers. For example, for a hollow ringtransducer, the capacitance is directly proportional to the height ofthe ring and inversely proportional to the natural logarithm of theratio of outer to inner diameters of the hollow cylinder, respectively.For a circular disk transducer which is polarized in either planar orthe thickness direction, the capacitance is directly proportional to thesquare of the diameter and inversely proportional to the thickness.These transducers are best utilized in such manner that their resonancematches the mechanical resonance of the test cell enclosure. For the 70mm OD and 150 mm long test cell shown in FIG. 1 the mechanical resonancefrequency (when filled with acetone) amounts to around 20 kHz and thecapacitance of the ring transducer is around 20 nF. For the disktransducers of FIG. 1 b, the disk transducers should be selected for the70 mm OD test cell with a capacitance around 20 nF as well but withdimensions selected to provide a resonance frequency of around 20 kHz.For larger diameter systems the mechanical resonance will roughly varyinversely with the ratio of diameters of the systems involved to a goodfirst order approximation (e.g., for a system with a diameter of 140 mmOD, the mechanical resonance thus may be expected to drop down toapproximately 10 kHz) and the capacitance of the transducers musttherefore, be adjusted accordingly to bring the resonance frequency ofthe transducer to become close to 10 kHz as well, such that maximumefficiency of drive power is attainable. A more refined estimate ofmechanical resonance of the system—one that includes multi-dimensional3-D effects, may be estimated via direct pressure mapping of the testcell at various elevations in the test liquid over a range offrequencies wherein, one would readily find the frequency at whichpressure oscillations reach their highest levels. Alternately, amulti-physics modeling and simulation scheme may also be employed, asshown later with use of the COMSOL®-multiphysics simulation platform.

Transducers can be affixed to the chamber using a coupling agent such asan adhesive. Suitable coupling agents will have a suitable impedancewhich essentially matches the product of density and sound velocity inthe medium to the driver transducer and the driven structure receivingthe mechanical impulses from the high frequency oscillating transducers.The coupling agent is chosen so that it minimizes acoustic energyscattering and/or wasting, such as by dissipation into heat. As anexample, epoxy can be used to affix the transducers to the chamberwalls. Trapped bubbles which adversely affect the coupling are to beavoided when using epoxy. Glass frit can be mixed in with the epoxy toimprove the coupling impedance but the quantity of glass used must belimited so as not to overly weaken the epoxy bonding properties. Anadhesive casting product, such as STYCAST® can also be used to affixtransducers. Metals that are liquids at room temperature, such asgalinstan—a eutectic mixture of Ga, In and Sn or other fluids withextremely low vapor pressure, such as tetradecane or glycerine can alsobe used as coupling agents. Acoustic energy is transmitted readilythrough such agents. The edge of the cavity holding such fluids thattransmit acoustic energy from the transducer to the glass wall may besealed around the edge with epoxies or silicon rubber (RTV-like)cements.

Many fluids may find use in the ATMFD such fluids include acetone,freon, benzene, isopentane, trimethyl borate, water, and the like, areall contemplated for use in the present invention. Detector fluidshaving higher hydrogen contents can be used to increase the effect ofdown scattering on the neutron flux ratio which in turn may enhance theresolution of directionality determination at large standoff.

The ATMFD devices can be refurbished such as by replacing transducers orrefilling the fluid in the gap between the chamber and the transducer.In the process care must be exercised during removal of epoxy or duringrefilling so as not to destroy the PZT transducers which can easilycrack or malfunction.

To operate the device a sinusoidal signal amplified by a linearamplifier can be used to drive the piezoelectric transducer which can bepolarized in the radial or axial direction. A piezoelectric materialwhen extended in a given direction will shrink in the direction at rightangles to that first direction. The need for driving perturbationslargely in the radial direction was the reason for choosing hollowcylindrical PZT transducers polarized in the radial direction; theelectrode leads are on the inner and outer surfaces of the rings.Vertically polarized hollow cylindrical PZTs are also available wherethe electrode leads are on the lips and may also be used. Alternatively,banks of flat disk piezoelectric transducers may also be affixed(mechanically or via glue/epoxy) to the glass surface and then drivenindividually or in parallel. Such a bank of drive transducers serve adual purpose; first, to provide drive power to the ATMFD, and then, alsoto serve as pickup devices for the shock signals arriving fromcollapsing cavitation bubbles; the shock signals being superimposed onto the main drive frequency. In such a case, the need for additionalsmall microphones is dispensed with, or alternately, may be used toderive more acoustic information related to ATMFD performance anddirectionality monitoring. When in resonance, the mechanical deformationof the quartz/glass/ceramic/metal chamber of the present dimensions canbe used to generate a standing acoustic wave consisting of oscillatingpositive and negative (i.e., sub-vacuum) pressures in the 20 kHz range.During the time the fluid molecules are under negative pressure, thestate is metastable whereupon, nuclear particle strikes from incidentradiation can be generated.

Modeling and Simulation

Two simulation tools can be used in the characterization of the ATMFDsystem: COMSOL® multiphysics a finite element multiphysics program andMCNP5—a nuclear particle transport code. COMSOL® allows for the couplingof acoustic, fluid, and structural models of the resonant acousticsystem. MCNP can be utilized to evaluate the combined spatial and energydependent physical aspects effecting the neutron flux and energyspectrum over the sensitive volume of the ATMFD.

Finite Element Simulation

A model of the resonant acoustic chamber can be developed utilizingCOMSOL's® structural mechanics module including stress-strain andpiezoelectric effects analysis and the acoustic wave transport moduletogether with electromagnetic coupling. Due to the complexity of theproblem, the COMSOL® model utilizes finite element methods to solve theproblem in the frequency domain.

Similar models using ethylene glycol and acetone as the host liquid havebeen benchmarked against experimental data for the pressure distributionand frequency spectra response. The ATMFD system was modeled asaxi-symmetric, symmetric about a central axis. The detector fluid usedin the model was pure acetone at about 25° C. FIG. 2 shows therelationship between variations in the drive power applied to the PZTand the spatial characteristics of the sensitive volume of the chamberat the resonant frequency of about 18.78 kHz. In the current detectorconfiguration, the sensitive volume of the chamber can be defined as thevolume of the chamber in which the oscillating negative pressurefluctuations are at or below −3.5 bar, which is the threshold negativepressure for detection of fast (MeV) neutrons in acetone.

As can be seen from FIG. 2, a modest doubling in drive power from about4.5 W to 10 W resulted in a linear increase of the sensitive volume fromabout 50 cm³ to about 100 cm³. Larger sensitive volumes in the detectornot only increase the effective detection efficiency, allowing for moreneutrons to interact in the sensitive volume of the chamber, but alsoincrease the radial dimension of the sensitive volume which can be usedto increase the resolution of directional information and increasedetection efficiency.

Monte Carlo Simulation

Nuclear particle transport assessments can be performed using the MCNP5code developed at Los Alamos National Laboratory, New Mexico, USA. Themodel consists of the ATMFD's resonant chamber and a PuBe neutron source(emitting about 2×10⁶ n/s) at a distance of about 20.3 cm from thecentral axis of the chamber. The chamber can be modeled asaxi-symmetric. Suitable structural materials including the reflectorscan be quartz, the piezoelectric transducers can be lead zirconatetitanate (PZT), and the working fluid can be acetone (C₃H₆O). Theportion above the top reflector, inside the top and bottom reflectorsand outside the chamber can be modeled as air.

According to this method two regional sensitive volumes are defined inthe detector fluid as seen in FIG. 3. The cylinder (r=about 1.25 cm andh=about 4 cm) defining the sensitive volume can be divided into twohalves; one half facing the source (V1) and one half facing away fromthe source (V2). The two half cylinders formed the neutron tallyvolumes. The neutron energy spectrum for a bare PuBe source can be usedand all cross sections are evaluated at about 300 K. All assessments canbe calculated to within about 1% relative error.

Results of MCNP5 simulations demonstrate about a 23% higher neutron fluxin V1 relative to V2. By comparing the solid angles subtended by the twosensitive volumes to the PuBe source, the spatial effects by themselvesresult in −13% higher flux in V1 than V2. In comparison, down scatteringaccounts for about a 10% higher fast neutron flux in V1. This shows thatas the source to detector distance increases, effectively reducing andeventually nullifying any possible contributions from spatial effects,the detection in the ATMFD will still be preferential to and discernableas favoring the region nearest the source.

Calculations based on the exponential attenuation law allow for aquantitative estimate of the relationship between the effect of downscattering on the neutron flux ratio and the size of the sensitivevolume. A second fluid, Freon-113 (commonly used in tension metastablefluid based detector systems), is included for comparison. The PuBesource emits neutrons with an average energy of about 4 MeV which have amean free path, λ, of about 5 cm in acetone and about 10 cm inFreon-113. The results are shown in FIG. 4. The effects of downscattering on the neutron flux ratio increases proportionally with thesensitive volume size. The ability to increase the amount of directionalinformation available is observed when the source is far enough awaysuch that solid angle effects on the magnitude of the neutron flux aresmall. The effects of down scattering are also dependent on thecomposition of the detector fluid. As seen from FIG. 4, down scatteringhas a larger effect in acetone compared to Freon-113. This is primarilyattributable to the higher hydrogen content (and therefore lower λ) ofMeV neutrons in acetone. Different detector fluids having higherhydrogen contents can be used to increase the effect of down scatteringon the neutron flux ratio which in turn may enhance the resolution ofdirectionality determination at large standoff. Thus, fluids such asacetone, freon, benzene, isopentane, trimethyl borate, water and thelike are all contemplated for use in the present invention.

Automation

The ability to decipher directionality requires acquisition of hundredsto thousands of detection signals and then rapid analysis to then resultin an answer with high enough confidence (e.g., >75%) within seconds tominutes. Automation is highly desirable for practical systems. Theviolent collapse of the imploding vapor cavity formed by a nuclearparticle interaction causes an audible click that can be heard severalfeet away from the chamber. The audible clicks from the collapsing vaporcavities can be recorded using four tiny MHz response piezoelectrictransducers which can be affixed to the outside of the resonant chamber.Hardware and LABVIEW® based virtual instrument software based controlsystems are developed to record these detection events and to extractinformation on the direction of the radioactive source.

The electrical signals from the piezoelectric transducers can be sentthrough a third order Butterworth high pass filter to eliminate thedominate drive frequency therefore isolating the high frequencycomponents. The signals from the filter can then be sent to an Agilent™100 MHz digital storage oscilloscope for display, storage, and furthersignal processing. Screen shots of actual signals used in the experimentare shown in FIG. 5 and FIG. 6. The peaks in the signal are results ofrecorded neutron detection events which resulted in imploding vaporcavities thereby radiating shock signals that are detected by the PZTtransducers.

A LABVIEW® based virtual instrument (VI) can be designed as a graphicaluser interface to not only control the operation of the ATMFD system butalso collect and analyze experimental data. Experimental data can beacquired by the LABVIEW® virtual instrument via a GPIB interface withthe oscilloscope. In one method once the data are acquired from theoscilloscope, the virtual instrument used two statistical techniques tovalidate the acquired electrical signal as a neutron detection event.Validation of the neutron detection events can take advantage of twocharacteristics of the shock traces. As can be seen from FIG. 6, theshock traces from neutron detection events have a high frequency (about250 kHz) sinusoidal pulse shape. A further characteristic of the shocktraces is that they are symmetrical. A measure of the symmetry of theshock traces, known as the skewness, can be calculated to determine ifthe electrical signal is that of a neutron detection event. Thistechnique takes advantage of the random nature of noise (both electricaland mechanical) and helps eliminate false positives. The secondtechnique that can be used is a measure of the similarity of two of theshock traces. In the case of a true neutron detection event, therecorded shock trace as seen by each transducer shock monitor shouldhave substantially the same shape. The cross-correlation, or sliding dotproduct, of two of the shock traces can be calculated to ascertain howwell the two individual shock traces match. The cross-correlation methodalso allows for the calculation of the time difference of arrivalbetween the two shock traces. The time difference of arrival (r) isshown graphically in FIG. 6. The time difference of arrivals can then beanalyzed with a hyperbolic positioning algorithm to calculate thelocation of the neutron detection event in the resonant chamber. TheLABVIEW® virtual instrument then utilizes the locations of the neutrondetection events to ascertain the direction of the neutron source anddisplay it graphically to the user. The integral detection systemdesigned for the ATMFD, from neutron detection event to ascertaining thedirection of the neutron source, can be performed in near real time onthe millisecond (ms) time scale.

Directionality Determination Experimentation and Results

The experimental setup utilizes preferably, a quartz ATMFD chamber withOD of about 6.9 cm although, other shapes such as spheres and conicaltypes made of Pyrex glass have also been tested successfully, asdescribed in MCNP and COMSOL® models. The liquid used in the chamber waspure acetone at about 25° C. and under about 20 inches Hg of vacuum. Thechamber was operated with a wave-form generator (Agilent, model 33120A)and a linear amplifier (Piezo Systems, Inc. model EPA-104). The resonantfrequency was found at about 18.3 kHz, and the drive voltage used wasabout 96 V. Experimental data was taken utilizing an oscilloscope whichrecorded the shock traces. The LABVIEW® program was used to control theoperation of the oscilloscope, collect data, and perform signalprocessing and analysis. Communication with the oscilloscope wasaccomplished via a GPIB interface. Four piezoelectric transducers wereplaced at right angles to each other on the same XY plane, with theexception of the fourth transducer which was placed with a positive Zcomponent to allow for 3-D positioning. The setup is shown in FIG. 7.

Experimentation was performed with about a 1 Ci PuBe neutron-gammasource (emitting about 2×10⁶ n/s) located about −20.3 cm and 20.3 cmfrom the center of the chamber on axis with Mic 1 and Mic 3. The TDOAsrecorded were used to calculate the positions of the nucleation eventsand are shown in FIG. 8. Due to the high Q-factor of the system, smallvariations in the construction of the chamber result in a smallvariation between the geometrical center and the center of the sensitivevolume that must be taken into account. The average position of thenucleation events in the XY plane was used to ascertain the center ofthe sensitive volume. The positions were adjusted accordingly. Thegraphs were first divided into two substantially equally sizedsemi-hemispherical volumes. The volume closest to the radioactivesource, V1, contained about 55.2% (±2.5%) of the detection events, andonly about 44.8% (±2.2%) of the detection events occur in V2. Theresulting ratio of neutron detection events is given as about 1.23(±0.07). As mentioned previously, the predicted ratio given by the MCNPmodel is about 1.23.

Further analysis of the positions of the neutron detection events wasperformed to determine the ability of the detector to better resolve theangular direction of the radioactive source. The sensitive volume wasdivided into 6 separate about 600 angular sectors. The total number ofneutron detection events in each sector was calculated and compared tothe number of neutron detection events occurring in the opposing sector.Similarly, a cylindrical mesh tally of the neutron flux in the sensitivevolume was added to the MCNP simulation. When comparing opposingsectors, the sector nearest the radioactive source was observed tocontain about 57.8% (±4.5%) of the neutron detection events, and onlyabout 42.2% (±3.7%) of the neutron detection events occur in the sectorfurthest away from the radioactive source. The resulting ratio ofneutron detection events was given as about 1.37 (±0.13) which againcorrelates with our prediction based on simulations of about 1.38. Againit is noted that the experimental results correlated to the theoreticalmodel estimates within one standard deviation.

A study was performed to investigate the ability of the ATMFD system todetect the direction of a radioactive source positioned in an unknownlocation. The opposing sector ratios for all sectors were calculated andplotted in FIG. 9 in order to correctly determine the correctradioactive source direction. It was exceptionally clear that the sectorpointing in the direction of the source had the highest opposing sectorratio. Logically, the second highest opposing sector ratio occurs in thesector directly adjoining the source direction sector. The sector ratioin the adjoining sector was given as about 1.16 (±0.11). The ratiopredicted by the MCNP simulation is about 1.18, within one standarddeviation of the experimental result. Analysis of the results shows thatthe ATMFD system is capable of locating the direction of the radioactivesource to within 30° with about 80% confidence.

Based on the principle that a higher pressure amplitude in the liquidprovides a higher probability that a neutron induced nucleation eventwill occur, the pressure field inside the chamber was mapped by thedistribution density and profile of neutron-induced bubble nucleationsites. Experimentation was done with the PuBe source about −20.3 cm and20.3 cm from the center of the chamber on axis with Mic 1 and Mic 3 toprevent the directional nature of the detector from becoming a factor.The positions of the detection event sites were plotted in the RZ planeand overlaid on top of the sensitive volume pressure field predicted bythe COMSOL® model. The results are shown in FIG. 10. Analysis of theresults showed that the neutron induced detection events primarilyoccurred at pressures lower than about −4 bar, which correlates with thepreviously measured threshold of about −3.5 bar. It was also apparentthat substantially all of the neutron detection events occurred within aradius of about 1.25 cm from the centerline of the ATMFD. Therefore thevalue of about 1.25 cm was used for the MCNP assessments.

This work demonstrates directionality determinations and also shows thatthe ATMFD system can be tailored to be insensitive to gamma radiationand that by changing the detector liquid to Freon-113 and trimethylborate the ATMFD system can also be simultaneously used to detectneutrons with energies spanning eight orders of magnitude whileoperating with nearly 90% intrinsic detection efficiency. This isenabled via (n, p) reactions with C1 atoms of freon, and, (n, alpha)reactions with boron atoms in trimethyl borate.

A method for determining the direction of incoming radiation in nearreal time is described. The experimental evidence presented herein hasshown that the locations of the neutron detection events occurpreferentially on the side of the detector nearest the source with aratio of about 1.23 (±0.07):1 which corresponds with our Monte Carlobased simulations (about 1.23:1). Calculations have been performed whichshow that the increase in solid angle from the sensitive volume nearestthe source to furthest from the source accounts for about a 13%reduction in neutron flux. The down scattering of the neutrons throughthe acetone accounts for about a 10% reduction. The directionalinformation may be intrinsically obtained with the ATMFD technology evenwhen the source is far enough away such that solid angle effects on themagnitude of the neutron flux are negligible. These same calculationsprove that the reduction of the neutron flux due to down scatteringincreases as the sensitive volume increases, therefore providing anavenue for increasing the accuracy and precision of the determination ofthe source direction at large standoffs. The COMSOL® coupled physicssimulation is benchmarked with experimental neutron detection data andcan have the ability to scale the sensitive volume of the detector byincreasing the drive power therefore yielding increased accuracy andprecision of the determination of the source direction and enhancedeffective detection efficiency.

Further analysis of the locations of the neutron detection events canyield improved methods of directionality determination via opposingneutron flux sector ratios. The results show that the ATMFD system, inits current configuration, is capable of locating the direction of aradioactive source to within about 30° with about 80% confidence.

A Second Embodiment

In another embodiment, tension metastable fluid states offer a potentialfor advancements in radiation detection. Such metastable fluid statescan be attained using tailored resonant acoustics to result in acoustictension metastable fluid detection (ATMFD) systems. Present-day neutrondetectors sometimes may be bulky, expensive, require different detectorsystems for various neutron energy groups and are not suited forproviding information on which direction neutron radiation arrived.Radiation detection in ATMFD systems is based on the principle thatincident nuclear particles interact with the dynamically tensioned fluidwherein the intermolecular bonds are sufficiently weakened such thateven fundamental particles can be detected over eight orders ofmagnitude or more in energy with intrinsic efficiencies far aboveconventional detection systems. In the case of neutron-nucleiinteractions the ionized recoil nucleus ejected from the target atomlocally deposits its energy, effectively seeding the formation of vapornuclei that grow from the sub-nano scale to visible scales such that itbecomes possible to record the rate and timing of incoming radiation(neutrons, alphas, and photons). Nuclei form preferentially in thedirection of incoming radiation. Imploding nuclei then result in shockwaves that are readily possible to not only directly hear but also tomonitor electronically at various points of the detector using timedifference of arrival (TDOA) methods. In conjunction with hyperbolicpositioning, the convolution of the resulting spatio-temporalinformation provides for the first time not just the rate of incidentneutron radiation but also its directionality.

The development of intrinsic-efficiency, low-cost, and rugged, ATMFDsystems can be accomplished using a combination ofexperimentation-cum-theoretical modeling. Modeling methodologies includeMonte-Carlo based nuclear particle transport using MCNP5, and alsocomplex multi-dimensional electromagnetic-cum-fluid structuralassessments with COMSOL's® multiphysics simulation platform. ATMFDsystem automation was accomplished with the programming of virtualinstrument (VI) control algorithms using LABVIEW® software.

Liquids like solids can withstand tension (i.e., they can sustainsub-vacuum pressures before tearing apart). A liquid in a tensile stateis metastable below its thermal equilibrium state, unlike a metastableliquid in a state of thermal superheat which is above its normal boilingpoint. Tension in fluids is analogous to the stretching (versuscompression) of solid structures. The energy required to tear apart theintermolecular bonds of a solid decreases as the structure is stretched.In an analogous manner, the energy required to break the bonds betweenliquid molecules decreases with increasing tension metastability;eventually resulting in spontaneous triggering of explosive phase changeat the spinodal (thermodynamic stability) limit of tension.

Explosive phase change can be triggered in metastable liquids below thestability limit. This triggering causes explosive vaporization of fastnucleating and expanding vapor pockets. The three possible methods oftriggering explosive phase changes in metastable liquids are laserheating, nuclear particle (e.g., neutrons) knock-on collisions andacoustic energy. The following discussion will focus on triggering bymeans of neutron-nuclei knock-on collisions. Explosive phase changes canthus be initiated mechanically or via nuclear particles or photons froma laser. The rapid, pulsed energy deposition resulting from knock-oncollisions between high energy particles, specifically neutrons, andindividual nuclei of liquid molecules can cause nanoscale triggering andexplosive phase change. The pulsed energy deposition of recoils fromknock-on collisions is in the form of thermal energy and is depositedover about a few nanometers causing a vapor nucleus to form. The rangein which the energy is deposited depends on the stopping power of therecoil ion in the liquid. If the thermal energy deposition rate is highenough to cause a vapor nucleus larger than the critical size thenucleus will continue to grow into a macroscopic vapor bubble. Criticalradii are generally in the nanometer range and are reached innanoseconds. Photons from a laser source can also be used to triggerexplosive phase changes though more are needed because individualphotons of visible light have a relatively small amount of energy (about1 eV) and less linear energy transfer (LET) compared to fast (MeV)neutrons.

For example, an approximately 4 MeV neutron colliding with a carbon atomin acetone will, on average, transfer about 0.72 MeV to the carbonnucleus. This gives an energy density, where the volume is defined bythe critical radius of an acetone vapor bubble (about 30 nm), of about36.4 MJ/kg. In comparison, the latent heat of vaporization for acetoneis about 0.534 MJ/kg. A single photon from a blue laser (about 400 nm)with an energy of about 2.48 eV has an energy density of about 9.6×10⁻⁷MJ/kg. In the case of light photons the volume is defined by thewavelength of the light photon. Thus, it takes interactions from about1.3×10⁹ blue light (UV) photons to equal the energy density of oneneutron knock-on collision.

The detection of nuclear particles from tensioned metastable statesrequires the induction of appropriate levels of negative pressures. Thisis distinct from that for the famous “bubble-chamber” as used for thesuperheated droplet detector (SOD) where the liquid is put above itsboiling point. In the system according to embodiments of the presentdisclosure, the liquid remains at room temperature. The principle ofdetection for fluids in tension metastability is based on an analogywith stretching of structures. The greater the degree of tension theeasier it becomes to tear the bonds holding the material together. In ananalogous manner, the greater the degree of negative pressure impartedto the molecules and atoms of the working fluid, the easier it becomesto tear the bonds holding the molecules together (i.e., to then causelocalized bubbles to form which can grow from nanometers to relativelylarge, multi-mm size pockets before redissolving on implosion). Thesensitivity of detection is based on the degree of imparted tension andthe value of spatial energy deposition from a given incident nuclearparticle, or, dE/dx.

ATMFD Design

Another embodiment of the ATMFD system, shown schematically in FIGS. 11and 12, is a resonant acoustic system comprised of a (approximately 60mm OD, 150 mm long) cylindrical glass, preferably Pyrex glass, resonantchamber powered by a concentrically affixed ring shaped piezoelectrictransducer. A sinusoidal signal amplified by a linear amplifier drivesthe piezoelectric transducer. Reflectors placed at the top and bottom ofthe chamber aid in energy focusing by the formation of a standingpressure wave. In this embodiment four (approximately 7 mm OD)disc-shaped piezoelectric transducers were fixed to the outer wall ofthe cylindrical portion of the chamber and used to detect the shock wavespectra generated by radiation induced cavitations occurring in thesensitive volume of the detector.

The sensitive volume of the ATMFD is defined as the region in which themagnitude of the tension (negative) pressure is below a certainthreshold value for which critical size vapor nuclei can be formed viaenergy deposition by incident nuclear particles colliding with themetastable state molecules.

Modeling and Simulation

Two simulation tools can be used in the characterization of the ATMFDsystem: COMSOL® multiphysics a finite element multiphysics program andMCNP5—a nuclear particle transport code. COMSOL® allows for the couplingof acoustic, fluid, and structural models of the resonant acousticsystem. MCNP can be utilized to evaluate the combined spatial and energydependent physical aspects effecting the neutron flux and energyspectrum over the sensitive volume of the ATMFD.

Finite Element Simulation

A numerical model using COMSOL® which is based on finite elementmethods, can be developed for frequency domain analyses and the resultsfrom the model can be compared with experimental data. The multiphysicsmodel set here utilizes COMSOL's® structure mechanics module includingstress-strain and piezoelectric effects analysis and the acoustic wavetransport module together with modeling of electromagnetic coupling.

We assume that the system is axi-symmetric. In an embodiment thedetector liquid is chosen to be pure acetone at about 25° C. The valuesfor various properties of acetone are listed in Table 1.

TABLE 1 PROPERTIES OF ACETONE AT 25° C. Density (kg/m³) 0.786 × 10³Viscosity (Pa s) 0.308 × 10⁻³ Sound velocity (m/s) 1174 Bulk viscosity(Pa s) about 1.5 × 10⁻³

A similar model, which uses ethylene glycol as the liquid, has beenbenchmarked against experimental data for the pressure distribution andfrequency spectra response. In the present embodiment, we keep the samephysical domain settings and boundary conditions of the benchmarkedmodel; changing the properties of the liquid and the structure of thesystem and introducing structural fluid damping.

Due to the large variations of dimensions between structure and liquid,the maximum relative size of mesh finite elements in liquid and solidregions are about 0.003 and about 0.017, respectively. The meshedstructure embodied a total of about 5237 elements. Numerical convergencehas been checked by use of finer meshing (about 20948 elements).

In order to visualize the relation and accompanying variations betweenthe power driving the system and the sensitive volume in the chamber,the oscillating pressure distributions are plotted in FIG. 11 at theresonant frequency of about 18.85 kHz.

As shown in FIG. 11, the sensitive volume of the ATMFD can be varied byvarying the drive voltage. Various other options also become feasible(e.g. using higher modes or superposition). Such a modeling approach canbe utilized for designing and devising an as-needed ATMFD with desiredlevels of detection sensitivity along with ability to derivedirectionality information.

Monte Carlo Simulation

A system model can be developed for nuclear particle transportassessments using the MCNP5 code and shown in FIG. 12. It consists ofthe ATMFD's resonant chamber and a PuBe neutron source (emitting about2×10⁶ n/s). The chamber is substantially axial-symmetric. All structuralmaterial including the reflectors can be quartz glass, the piezoelectrictransducers can be lead zirconate titanate (PZT), and the representativedetection fluid can be acetone (C₃H₆O). The portion above the topreflector, inside the top and bottom reflectors and outside the chamberis modeled as air.

Two regional sensitive volumes are defined in the detector fluid as seenin FIG. 12. The cylinder (r=about 1.5 cm and h=about 4 cm) defining theentire sensitive volume is divided into substantially two halves; onehalf facing the source (V1) and one half facing away from the source(V2). The two half cylinders form the neutron tally volumes.

The neutron energy spectrum for a bare PuBe source is used and all crosssections are evaluated at about 300 K. The source is placed about 35.5cm from the central axis of the chamber to be consistent with theexperimental configuration.

Results of MCNP5 simulations demonstrate about a 25% increasedprobability of neutron interactions in the direction of the incidentneutron source. This result confirmed estimates from first principleestimates.

By comparing the solid angles subtended by the two sensitive volumes tothe PuBe source, one can see that the spatial effect for neutron fluxamounts to about a 15% higher flux in V1 than V2. In comparison, downscattering accounts for about a 10% higher fast neutron flux in V1. Thisindicates that even if the source is further away, effectively negatingthe solid angle dependence of the neutron flux, detection in the ATMFD(as presently disclosed) will still be preferential and discernable asfavoring to the side nearest to the source.

Automation

The collapse of the imploding vapor cavity formed by a nuclear particleinteraction causes an audible click that can be heard several feet awayfrom the chamber. The audible clicks from the collapsing cavities can bereadily recorded using the piezoelectric transducers which can beaffixed to the outside of the chamber. Recording the time that the shockwave reaches each transducer then allows the time difference of arrival(TDOA) to be calculated. The TDOA between transducers can be used inconjunction with a hyperbolic positioning algorithm to calculate theactual position of the bubble cavitation events.

The electrical signals from these cavitation events can first be sentthrough a third order Butterworth high pass filter to eliminate thedominate drive frequency therefore isolating the high frequencycomponents. The signals from the filter can then be sent to anoscilloscope, such as an Agilent™ 100 MHz digital storage oscilloscope,for display, storage and further signal processing. A screen shot of theactual signals used in the experiment is shown in FIG. 13.

The first channel shown in FIG. 13 is the unfiltered transducer signal.The second channel is the corresponding signal after the high passfilter. The peaks in the signal are the unmistakable results of arecorded cavitation pulse. A LABVIEW® program was created as a graphicaluser interface with the oscilloscope. Using the LABVIEW® program theoscilloscope was set to run until the analog signal on the triggeringchannel crossed a predetermined threshold level. From the screenshot ofthe oscilloscope, several measurements are acquired; XatMaxY (time in usof the highest peak relative to the trigger point), XatMinY (time in μsof the lowest peak relative to the trigger point), and Maximum (maximumvoltage recorded in the screenshot). Actual screenshots of typicalsignals used are shown in FIG. 14.

The measurements XatMaxY and XatMinY serve two purposes. Thesemeasurements allow for the calculation of the TDOAs between signals andfor an estimation of the frequency of the cavitation pulse as recordedby each transducer. The maximum, voltage measurement ensures that theheight of the cavitation signals on all four channels is larger than thetriggering level. The values of the TDOAs, frequency, and maximumvoltage of each cavitation pulse as recorded by the transducer were usedas constraints to determine whether the signal analyzed is that of acavitation pulse.

Data Constraints

The TDOA constraint was set using a numerical analysis of a hyperbolicpositioning algorithm. A LABVIEW® computer program can be used togenerate a random sample of cavitation events inside the chamber. Thecavitation positions are then used to calculate the TDOA that eachtransducer would record. The TDOAs are then analyzed with a hyperbolicpositioning algorithm. Upper constraints are set on the TDOAs used inthe data set to investigate what TDOAs would result in a cavitationposition mapped outside of the modeled sensitive volume of the chamberas mentioned earlier. The results are shown in Table 2. Therefore theupper constraint for TDOAs of about 20 μs resulted in cavitations withinabout 2 cm of the central axis of the chamber, and is consistent withexperimental findings.

TABLE 2 DATA CONSTRAINTS USED FOR TDOA CALCULATIONS TDOA ConstraintMaximum Radius of Zone 15 μs 1.57 cm 20 μs 1.97 cm 40 μs 2.95 cm

The dominant frequency of the largest peaks in the cavitation pulse canbe determined using the XatMaxY and XatMinY measurements. As seen inFIG. 14, the XatMaxY and XatMinY measurements should occur at the peaksof the cavitation pulse with the greatest magnitude. The frequencyconstraint can be experimentally investigated using LABVIEW® softwareand the oscilloscope. A LABVIEW® program can be designed to recordsubstantially the entire analog waveform of a cavitation pulse. Anexperimental data set of about 100 cavitation based neutron detectionevents can be recorded for analysis. Fast Fourier Transforms are thendone on the cavitation waveforms. It was found that the dominantfrequency of the largest peaks in the cavitation pulse is about 300 kHz.A lower constraint of about 200 kHz can be used to determine whether ornot the signal recorded contained a cavitation pulse. A large range offrequencies are accepted because the recorded cavitation frequenciesvary according to the cavitation strength, the distance to the recordingtransducer, the frequency response of the transducer (due tomanufacturing), and the level of dampening in the chamber (due toscattering centers e.g. vapor or gas bubbles).

The maximum voltage measurement allowed for a minimum voltage constraintin order to eliminate triggering bias was discovered in initialexperiments done by hand calculations and is explained later. Theminimum voltage constraint can be set using experimental data. Themaximum voltage identified from about 450 cavitations was recorded byall four transducers using a PuBe neutron source. In order to eliminateany maximum voltage bias due to source position, four sets of data aretaken with the source about +35.5 cm and −35.5 cm away from center ofthe chamber on the X-axis with transducers 1 and 3 and on the Y axiswith transducers 2 and 4. The average maximum voltages of thecavitations recorded are shown in Table 3.

TABLE 3 RECORDED AVERAGE MAXIMUM VOLTAGE MICROPHONE RESPONSE TOCAVITATION SHOCK EVENTS Mic 1 Mic 2 Mic 3 Mic 4 Source 1 716 mV 692 mV703 mV 667 mV Source 3 697 mV 702 mV 716 mV 661 mV Average 707 mV 697 mV710 mV 664 mV

The average maximum voltage of a cavitation pulse can be used to settrigger levels for each transducer. Only cavitations that have maximumvoltages larger than the trigger levels for all four transducers arerecorded for analysis. This method allows the oscilloscope toessentially trigger on all four signals at once, therefore eliminatingany trigger bias. The initial results indicate that the trigger levelsare within about 6% of each other, therefore the same trigger levelscould be used for each transducer. Early experimental results indicatethat the triggering levels used influence the accuracy of the results.To investigate the effect, a LABVIEW® program can be designed to recordthe rate of cavitations as recorded by each transducer as the triggerlevel is varied from about 5 mV to 195 mV. The results are shown in FIG.15.

The experimental results indicate that triggering rate was unstable forsmall triggering voltages up to about 100 mV and stabilized around about200 mV. Therefore a triggering level of about 200 mV can be used as abaseline for this embodiment. Due to variations in manufacturingtolerances and variability in use of epoxy or some other material toattach the pill microphones, the precise trigger level can now bedeveloped using the method just described.

Directionality Determination Experimentation and Results

The experimental setup utilized a quartz ATMFD chamber with diameter ofabout 6.9 cm, as described in MCNP and COMSOL® models. The liquid usedin the chamber was pure acetone at about 25° C. and under about 20 in.Hg of vacuum. The chamber was operated with a wave-form generator and alinear amplifier. The resonant frequency was found at about 18.3 kHz,and the drive voltage used was about 100 V. Data were taken utilizing anoscilloscope which recorded the shock traces. The LABVIEW® programmentioned earlier controlled the operation of the oscilloscope andcollected the data. Communication with the oscilloscope can beaccomplished via a GPIB interface. Four piezoelectric transducers wereplaced at right angles to each other on the same XY plane, with theexception of the fourth transducer which was placed with a positive Zcomponent to allow for 3-D positioning. The setup is shown in FIG. 16.

Data were first taken utilizing an oscilloscope which recorded thecavitation pulses. The TDOAs were recorded manually with theoscilloscope cursors. The TDOAs were then analyzed with a hyperbolicpositioning algorithm as mentioned earlier. The preliminary data (shownin FIG. 17), with the PuBe source about 13 cm away from the center ofthe chamber on the X axis, provided evidence that the location of thecavitation events in the chamber of the detector was convincingly biasedtowards the direction of the radioactive source.

The chamber can also be divided into two equally sized sections. Thesection closest to the source comprises about 65% (84/124) of thecavitation events, and about 32% (40/124) of the cavitation eventsoccurred in the section furthest away from the source. This resulted ina ratio of about 2.1:1 (2.1). It was also discovered that a bias towardsthe triggering signal occurs. This triggering bias occurs because thedata set was taken while triggering on only one signal. The triggeringbias occurs because the cavitations closer to the triggering transducerhave greater recorded shock signal amplitudes; therefore the cavitationsthat occur close to the triggering transducer may be preferentiallybiased. However, this trigger bias can be eliminated if all fourtransducers are used to trigger a cavitation event. This method ofrecording cavitation positions is inefficient, and taking data by handonly allowed for approximately 2 Sa/min to be recorded. Therefore, anautomation system that allows for processing of significant amounts ofdata was designed and used thereafter. However, the manually obtaineddata serves as confirmation of the ability to offer directionalinformation and also serves as a benchmark.

The data taken using the automated system are analyzed with thehyperbolic positioning algorithm mentioned earlier as well as with a LabVIEW LABVIEW® program designed to keep track of how many times eachtransducer recorded the cavitation pulse first. A count of how manytimes two of the transducers recorded the cavitations first is listed inTable 4.

TABLE 4 RESULTS TAKEN WITH PuBe SOURCE PLACED 35.5 cm FROM CENTER AXISOF CHAMBER ON-AXIS WITH MIC 1 AND MIC 3 Mic 1 Mic 3 Source at 1 88 65Source at 3 71 82

The chamber again can be divided into two equally sized sections. Thesection closest to the radioactive source contains about 56% (170/306)of the cavitation events, and about 44% (136/306) of the cavitationevents occur in the section furthest away from the source. The resultingratio of cavitation events is given as about 5:4 (1.25). These resultscorrelated with the results previously taken by hand, and with thetheoretical value given by the MCNP model (about 1.24). The differencebetween the counts taken by the computer and by hand can be attributedto the elimination of the trigger bias by setting a lower constraint forthe maximum voltage of the cavitations. The TDOAs recorded are also usedto calculate the positions of the cavitations and are in FIGS. 18 and19.

The graphs are also divided into two substantially equally sizedsections. The section closest to the radioactive source contains about56% (170/306) of the cavitations, and about 44% (136/306) of thecavitation events occur in the section furthest away from the cavitationsource. The resulting ratio of cavitation events is given as about 5:4(1.25). It is noted that the data using the first transducer of arrivalmethod and the hyperbolic position method correlate with about 100%accuracy, and also correlate to the theoretical model estimates (i.e.,MCNP5 and COMSOL®) to within about 2%.

A graph was also prepared which included all cavitation events recorded,with the radioactive source about −35.5 cm and 35.5 cm away from thecenter of the chamber on the X-axis with Mics 1 and 3. The graph of thecavitation events as seen in, the XZ plane shows that the sensitivevolume of the chamber was similar in size and shape to the developedCOMSOL® mode (FIG. 20). It can be seen that all neutron detection(cavitation events) occur within a radius of about 1.5 cm from thecenterline of the ATMFD, which corresponds very well with thepredictions of the COMSOL® model. Therefore, the value of about 1.5 cmis used for MCNP calculations.

The radial (spatial) distribution of the cavitation locations was alsoanalyzed. The cavitation events are distributed into substantially twoequal parts; the half of the chamber closest to the source, and the halfof the chamber furthest away from the source. The radial positions werethen tabulated in a histogram, which resulted in the number ofcavitations in opposing concentric arcs. The results are shown in FIGS.21 a and 21 b.

The center section of the chamber, which has the greatest tension(negative) pressure, was eliminated from this count, because thislocation is where cavitation events preferentially occur, and the errorin the hyperbolic positioning algorithm is the greatest. Therefore thecavitations that occurred in this section of the chamber contribute theleast to directional information. The resultant counts show that about56% (154/274) of the cavitation events occurred in the section closestto the source, while about 44% (120/274) occurred in the sectionfurthest from the source, equivalent to a ratio of 5:4 (1.28). Thismethod of data analysis provides for improved and better capability forderiving directional information, when compared to the simpler firsttransducer of arrival method.

A related means for determining directionality of incoming radiation ispossible from visual inspection of bubble shapes. It is found thatradiation from a source such as a Pu—Be source of neutrons deliversenergy from the radiation to the nuclei of atoms in the direction of thesource to give rise to nanoscale bubbles which grow to macroscopicvisible sizes. In an oscillating acoustically-driven field, the tinybubbles grow to macroscopic sizes in the multi mm range in the ATMFDsystems discussed earlier and then elongate themselves in elliptic shapetransporting themselves radially outward towards the glass walls viaacoustic pressure gradient prior to dissolving and disappearing. Themajor axis of the elongated bubble cluster (formed in an ATMFD system)from neutron induced collision with nuclei of acetone is pointingtowards and is in line with the neutron source. Observation of movieclips taken with a 1,000 fps camera as well as with a conventional 30fps video camera indicates that approximately 8 of every 10 of thebubble clusters point in this preferential direction. Some of theincoming neutrons striking the tensioned liquid molecules can beexpected to come as reflected neutrons from other angles, or also tostrike the nuclei of target atoms of the fluid at grazing angles, andhence, may be expected to give rise to elliptically transported bubblesin various other directions away from the true source of radiation.Nevertheless, this finding gives rise to the possibility for determiningdirectionality on a relatively instant (within seconds) reliable basisby direct visual image monitoring and analysis inspection of thetransient bubble clusters. Such a system would also become extremelyvaluable in situations involving very low intensity radiation arrivingat the detector (e.g., from well-shielded nuclear materials) whereby,use of the TDOA based technique becomes impractical for real-timemonitoring of directionality.

SUMMARY AND CONCLUSIONS

In both the first transducer of arrival and the hyperbolic positioningmethods used, the neutron detection as evidenced by location ofcavitation events (in an ATMFD with at about 70 mm OD) preferentiallyoccurs on the side of the detector nearest the source both with a ratioof about 1.25:1. Downscattering events in chambers of this size plays animportant role in permitting reliable discerning of directionality;larger ATMFDs can lead to even higher confidence levels and in lesstime. Therefore, it is discernable that the addition of the hyperbolicpositioning algorithm, which allows for mapping of the cavitation eventsin 3D, does not increase the error involved. The ability to map thecavitation events in three dimensions signifies that there is not onlythe ability to detect 2D directionality, but also 3D directionalityinformation.

Cavitation events are found to occur preferentially on the side of thedetector nearest to the source with a ratio of about 1.25:1 comparedwith predictions from the multiphysics based simulations (about 1.24:1).These ratios are for a source to detector distance of about 35.5 cm.Calculations confirm that for this distance the increase in solid anglefrom the sensitive volume nearest the source to furthest from the sourceaccounts for about a 15% reduction in neutron flux. This means that downscattering of the neutrons through the acetone (even for anapproximately 6 cm OD ATMFD system) accounts for a very significant(approximately 10%) effect. Larger ODs will allow for greater abilityfor directionality. Therefore, directional information may be obtainedeven when the source is far enough away such that solid angle effects onthe magnitude of the neutron flux are negligible.

Our COMSOL® coupled physics simulation shows the ability to scale thesensitive volume of the detector by increasing the drive power andyielding increased confidence directional information in less time thanthe baseline case.

ATMFD may be insensitive to gamma radiation and by changing the liquidto be composed of Cl or B nuclei (Freon-113 or tri-methyl borate) theATMFD can also be simultaneously used to detect neutrons/radiation withdirectionality with energies from thermal to fast and approximately 100%intrinsic efficiency has also been demonstrated for TMFD systems.

Radiation collisions with nuclei of TMFD systems deliver energy inpreferential directions that coincide to a large extent with thedirection of arrival on to the nuclei of atoms of liquid molecules ofthe TMFD system. Tension pressures amplify the bubbles from nanoscale tothe multi-mm scales in such way that bubbles can deform to elongated andapproximately cylindrical comet-like shapes with the major axissubstantially pointing in line with the direction of incoming radiation.

While the present disclosure has been described with reference tocertain embodiments, other features may be included without departingfrom the spirit and scope of the present invention. It is thereforeintended that the foregoing detailed description be regarded asillustrative rather than limiting, and that it be understood that it isthe following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

The invention claimed is:
 1. A method of determining directionality ofradiation comprising, creating a volume of a tensioned metastable fluidin a chamber; placing the tensioned metastable fluid volume in theproximity of a radiation source; detecting the location of radiationinduced cavitations within the tensioned metastable fluid; anddetermining the direction of the radiation source based on the radiationinduced cavitations within the tensioned metastable fluid; wherein thestep of detecting the location of radiation induced cavitationscomprises detecting the time delay of the arrival of cavitation inducedshock signals by processing signals obtained from a plurality of signaldetection transducers mounted on the chamber.
 2. The method ofdetermining directionality of radiation of claim 1, wherein the volumeof tensioned metastable fluid has a shape that contains at least oneaxis of symmetry.
 3. The method of determining directionality ofradiation of claim 1, wherein the tensioned metastable fluid is anacoustically tensioned metastable fluid.
 4. The method of determiningdirectionality of radiation of claim 1, wherein the step of detectingthe location of radiation induced cavitations comprises detectingcavitation induced shock signals by processing signals obtained from aplurality of signal detection transducers mounted on the chamber whereinthe processing further comprises a step to minimize bias.
 5. The methodof determining directionality of radiation of claim 1, wherein the stepof detecting the location of radiation induced cavitations comprisesdetecting cavitation induced shock signals by processing signalsobtained from a plurality of signal detection transducers mounted on thechamber wherein the processing further comprises a step to minimize biasthat includes the step of detecting signals from the signal detectiontransducers that are above a threshold voltage level, wherein thethreshold voltage level can be determined from an asymptotic responsecomparison of all transducers.
 6. The method of determiningdirectionality of radiation of claim 1, wherein the step of detectingthe location of radiation induced cavitations comprises the method ofdetecting the location of cavitations by a hyperbolic positioningmethod.
 7. The method of determining directionality of radiation ofclaim 1, wherein the step of detecting the location of radiationinvolves determining a ratio of cavitations occurring in at least tworegions of the chamber.
 8. The method of determining directionality ofradiation of claim 1, wherein the chamber has a centerline vertical axisand the method further comprises comparing cavitation events in opposingsectors of the chamber without including the event counts in a volume ofspace that includes at least a portion of the centerline vertical axis.9. The method of determining directionality of radiation of claim 1,wherein the method further comprises using pressure differences toamplify the elongation of cavitation bubbles to coincide with directionof energy transfer to liquid molecules from the incoming radiation. 10.The method of determining directionality of radiation of claim 1,wherein the method further comprises monitoring the shape of thecavitation bubble nucleation events.
 11. The method of determiningdirectionality of radiation of claim 1, wherein the step of determiningthe direction of the radiation source comprises the step of determiningthe major axis of an elongated shape of cavitation bubbles induced bythe radiation.
 12. The method of determining directionality of radiationof claim 1, wherein the method further comprises visually determiningthe direction of incoming radiation from the major axis of an elongatedcavitation induced bubble.
 13. A device for determining directionalityof incident radiation comprising: a chamber holding a fluid, a controlsystem in communication with a mechanism for deforming the chamber thatincludes at least one drive transducer and the resonance frequency ofthe at least one drive transducer is substantially similar to theresonance frequency of the chamber; wherein the control system and themechanism for deforming the chamber operate together to induce andmaintain a tension metastable state in the fluid that is sufficient toallow the nucleation of bubbles when the fluid molecules are struck byincident nuclear particles and a plurality of signal detectiontransducers spaced apart within the chamber in electronic communicationwith a system for determining the location of bubble cavitation eventswithin the fluid volume.
 14. The device for determining thedirectionality of incident radiation of claim 13, wherein the chamber issealed.
 15. The device for determining the directionality of incidentradiation of claim 13, wherein the fluid in the chamber is selected fromthe group of fluids consisting of acetone, freon, benzene, isopentane,trimethyl borate, and their mixtures.
 16. The device for determining thedirectionality of incident radiation of claim 13, wherein the mechanismfor deforming the chamber further comprises at least one transducer thatincludes a piezoelectric material.
 17. The device for determining thedirectionality of incident radiation of claim 13, wherein the mechanismfor deforming the chamber further comprises at least one transducer thatincludes a piezoelectric material comprising lead zirconate titanate.18. The device for determining the directionality of incident radiationof claim 13, wherein the mechanism for deforming the chamber furthercomprises at least one transducer that includes a piezoelectric materialcomprising ceramic.
 19. The device for determining the directionality ofincident radiation of claim 13, wherein the mechanism for deforming thechamber further comprises at least one transducer that includes apiezoelectric material comprising barium titanate.
 20. The device fordetermining the directionality of incident radiation of claim 13,wherein the mechanism for deforming the chamber includes at least onetransducer mounted to the chamber such that it surrounds thecircumference of the chamber around the mid plane or in a planecorresponding to a desired oscillating tension-cum-compression pressurefield.
 21. The device for directionality of incident radiation of claim13, wherein the mechanism for deforming the chamber includes multipletransducers mounted to the chamber at discrete locations in a planecorresponding to a desired oscillating tension-cum-compression pressurefield.
 22. The device for determining the directionality of incidentradiation of claim 13, wherein at least one drive transducer is anelectrically driven piezoelectric element mounted to the chamber wallwhich is capable of introducing positive and negative pressurefluctuations within the fluid to establish resonance and a tensionmetastable state that is sufficient to allow the nucleation of bubbleswhen the fluid molecules are struck by incident nuclear particles. 23.The device for determining the directionality of incident radiation ofclaim 13, further comprising at least four signal detection transducers.24. The device for determining the directionality of incident radiationof claim 13, further comprising at least three signal detectiontransducers in the same plane and at least one signal detectiontransducer that is outside that plane.
 25. The device for determiningthe directionality of incident radiation of claim 13, wherein the systemfor determining the location of bubbles within the fluid volume includea signal processing system comprising a high-pass filter circuit thatremoves the baseline drive frequency signal.
 26. The device fordetermining the directionality of incident radiation of claim 13,wherein the system for determining the location of bubbles within thefluid volume includes a signal processing system that compares filteredsignals from the signal detection transducers to determine the arrivaltime delay of bubble signals at the signal detection transducers thatemploys a positioning algorithm to determine position of implodingbubbles within the chamber.
 27. The device for determining thedirectionality of incident radiation of claim 13, wherein the system fordetermining the location of bubbles within the fluid includes a signalprocessing system that determines the number and location of bubblecavitations in the chamber.
 28. The device for determining thedirectionality of incident radiation of claim 13 wherein the chamber hasa size and shape that allows for the directional detection of radiationthat permits down scattering assisted collection of cavitation events invarious regions of the chamber.
 29. A device for determiningdirectionality of incident radiation comprising: a chamber holding afluid, a control system in communication with a mechanism for deformingthe chamber which operate together to induce and maintain a tensionmetastable state in the fluid wherein the tension metastable state issufficient to allow the nucleation of bubbles when the fluid moleculesare struck by incident nuclear particles and a plurality of signaldetection transducers spaced apart within the chamber in electroniccommunication with a system for determining the location of bubblecavitation events within the fluid volume comprising a signal processingsystem that determines the number and location of bubble cavitations inthe fluid.
 30. The device for determining the directionality of incidentradiation of claim 29, wherein the chamber is sealed.
 31. The device fordetermining the directionality of incident radiation of claim 29,wherein the fluid in the chamber is selected from the group of fluidsconsisting of acetone, freon, benzene, isopentane, trimethyl borate, andtheir mixtures.
 32. The device for determining the directionality ofincident radiation of claim 29, wherein the mechanism for deforming thechamber further comprises at least one transducer that includes apiezoelectric material.
 33. The device for determining thedirectionality of incident radiation of claim 29, wherein the mechanismfor deforming the chamber further comprises at least one transducer thatincludes a piezoelectric material comprising lead zirconate titanate.34. The device for determining the directionality of incident radiationof claim 29, wherein the mechanism for deforming the chamber furthercomprises at least one transducer that includes a piezoelectric materialcomprising ceramic.
 35. The device for determining the directionality ofincident radiation of claim 29, wherein the mechanism for deforming thechamber further comprises at least one transducer that includes apiezoelectric material comprising barium titanate.
 36. The device fordetermining the directionality of incident radiation of claim 29,wherein the mechanism for deforming the chamber includes at least onetransducer mounted to the chamber such that it surrounds thecircumference of the chamber around the mid plane or in a planecorresponding to a desired oscillating tension-cum-compression pressurefield.
 37. The device for directionality of incident radiation of claim29, wherein the mechanism for deforming the chamber includes multipletransducers mounted to the chamber at discrete locations in a planecorresponding to a desired oscillating tension-cum-compression pressurefield.
 38. The device for determining the directionality of incidentradiation of claim 29, wherein the mechanism for deforming the chambercomprises at least one electrically driven piezoelectric element mountedto the chamber which is capable of introducing positive and negativepressure fluctuations within the fluid to establish resonance and atension metastable state that is sufficient to allow the nucleation ofbubbles when the fluid molecules are struck by incident nuclearparticles.
 39. The device for determining the directionality of incidentradiation of claim 29, further comprising at least four signal detectiontransducers.
 40. The device for determining the directionality ofincident radiation of claim 29, further comprising at least three signaldetection transducers in a plane and at least one signal detectiontransducer that is outside that plane.
 41. The device for determiningthe directionality of incident radiation of claim 29, wherein the systemfor determining the location of bubbles within the fluid volume includesa signal processing system comprising a high-pass filter circuit thatremoves the baseline drive frequency signal.
 42. The device fordetermining the directionality of incident radiation of claim 29,wherein the system for determining the location of bubbles within thefluid volume includes a signal processing system that compares filteredsignals from the signal detection transducers to determine the arrivaltime delay of bubble signals at the signal detection transducers thatemploys a positioning algorithm to determine position of implodingbubbles within the chamber.
 43. The device for determining thedirectionality of incident radiation of claim 29 wherein the chamber hasa size and shape that allows for the directional detection of radiationthat permits down scattering assisted collection of cavitation events indistinct regions of the chamber.